The field of this invention is electrostatic printing, and more specifically a print head for charged particle generation.
Electrostatic printing, which is also referred to as ion deposition printing, charge deposition printing or electron beam imaging, has been used successfully for many years in a number of commercial embodiments. The apparatus and method disclosed in U.S. Pat. No. 4,155,093 to Fotland et. al., issued on May 15, 1979, form the basis of modern electrostatic printing technology. A flat print head is used in conjunction with a dielectric drum to create charge patterns on the drum, which attract toner particles. A piece of paper is then pressed into contact with the drum, acquiring the toner particles from the drum to receive a printed image.
Referring to FIG. 1, a typical flat print head 100 known in the art is shown. The flat print head typically includes two sets of selectively-controlled electrodes separated from one another by a high-strength first dielectric 108. The first set of electrodes 102, often referred to as driver electrodes 102, extend along the longer dimension of the print head, typically spanning the width of a page or other paper to be printed upon. The second set of electrodes 104, often referred to as finger electrodes 104, crosses the first electrodes obliquely. The driver electrodes 102 and the finger electrodes 104 form a matrix of crossing junctions between them, referred to as discharge sites 116. To create a charged particle discharge at a particular discharge site 116, a radio frequency (RF) signal of several thousand volts is applied to the driver electrode 102 at that discharge site 116. When a second charge is applied to the finger electrode 104 at that discharge site 116, charged particles are discharged at that discharge site 116 as a low energy spark or electric discharge. The print head 100 may be constructed to discharge either positive or negative charges. The negative charge may contain ions, electrons or a combination of both. The charged particles from a discharge site 116 cross a gap and impact a drum 112, where they are deposited on its dielectric surface 114. The print head 100 is configured such that the charge deposited by each discharge site 116 forms a dot-like latent charge image on the drum. Images or text can be created as aggregations of such dots. Thus, by controlling the discharge of particles from the matrix of discharge sites 116, and rotating the drum 112, images larger than the matrix of discharge sites 116 can be created and transferred onto paper or another surface.
In print heads of this type, RF-driven driver electrodes 102 are typically line conductors extending along the length of the print head, spanning a number of finger electrodes 104 which typically cross the driver electrode 102 at an angle. In an exemplary commercial embodiment, sixteen parallel driver electrodes 102 extend the width of a printed page, and they are crossed obliquely by 160 finger electrodes 104. A discharge site 116 is located at each point where a driver electrode 102 intersects a finger electrode 104. Each finger electrode 104 crosses the driver electrode 102 sixteen times, and can project up to sixteen charge dots, one from each discharge site 116 arranged along its length. According to Gauss"" Law, electric field lines originate perpendicular to a conducting surface. Theoretically, charge deposition follows those electric field lines, because electric force is exerted substantially along those field lines. In practice, charge eventually builds up on the drum 112, creating an electric field opposing the existing field. As a result of the presence of the opposing electric field on the drum 112, the charged particles will follow trajectories altered from the ideal trajectories perpendicular to the discharge surface, causing the charge to spread out. This is referred to as the blooming effect. The blooming effect becomes more severe as the distance between the print head 100 and the drum 112 increases, because the electric field generated by the print head 100 weakens with distance, subjecting the charged particles to increased influence from the opposing electric field exerted by the accumulated particles on the drum 112.
Because the print head 100 is flat and the drum 112 is cylindrical, the gap distance between the discharge sites 116 and the drum 112 is not uniform across the width of the print head 100. The electric field between the print head 100 and the drum 112 weakens as the distance between the discharge sites 116 and the longitudinal centerline of the print head 100 increases. In this document, the longitudinal direction is understood to be the direction of the axis of the drum 112. Consequently, the charge deposited on the drum 112 from the discharge sites 116 is not uniform across the width of the print head 100. As a result, the dots produced by the discharge sites 116 located further from the centerline of the print head 100 are weaker than those produced by discharge sites 116 at or near the centerline of the print head 100. This varying charge dot intensity caused by nonuniform charge deposition creates artifacts such as but not limited to smearing and venetian blinding in the image laid down by the drum 112. Venetian blinding is a defect well known to those skilled in the art, in which striations extending parallel to the direction of motion of the drum 112 appear in the image. These striations have different intensities of shading, directly correlating to the different charge intensities deposited on the drum 112 from the discharge sites 116.
A number of different attempts have been made to fix the image artifacts caused by varying charge dot intensity across the print head 100.
One category of attempts to solve the image artifact problem utilizes additional electrodes to better focus the charged particle beam. One or more additional sets of electrodes 106, generally referred to as screen electrodes 106, may be provided between the finger electrodes 104 and the drum 112. The screen electrodes 106 are apertured, and separated from the finger electrodes 104 by a second dielectric 110 having a number of cavities corresponding to the discharge sites 116 and the apertures in the screen electrodes 106. By applying a constant bias between the screen electrodes 106 and the drum 112, and a switchable bias between the screen electrodes 106 and the finger electrodes 104, the screen electrodes 106 act as lenses to improve image quality, and additionally act to prevent accidental erasure of deposited charges. The use of one or more sets of screen electrodes 106 in a print head 100 is described in, for example, U.S. Pat. No. 4,160,257; U.S. Pat. No. 4,675,703; U.S. Pat. No. 5,159,358; and U.S. Pat. No. 5,278,588. While the screen electrodes 106 can improve the quality of the printed image, the addition of one or more electrodes to the print head 100 increases the number of manufacturing steps required, and requires more parts which can malfunction or be damaged as the print head 100 is used. The added complexity of manufacturing also results in increased cost to the end user.
A second category of attempts to solve the image artifact problem modifies the discharge sites 116 or the dielectric material adjacent the discharge sites 116 to improve control over the charged particle stream emitted from the discharge site 116. Such modifications to the discharge site include angling the walls of the dielectric cavity adjacent each discharge site (U.S. Pat. No. 4,691,213; U.S. Pat. No. 4,683,482), providing a number of separate apertures at each discharge site (U.S. Pat. No. 4,879,569), and inserting dielectric material into the second electrode (U.S. Pat. No. 4,891,656). While the modification of the shape and configuration of each individual discharge site 116 can improve the quality of the printed image, the creation of complex discharge sites increases the complexity and cost of manufacturing the print head, resulting in higher manufacturing costs and higher costs to the end user.
A third category of attempts to solve the image artifact problem is disclosed in U.S. Pat. No. 4,819,013 to Beaudet. The print head of Beaudet has a semi-cylindrical surface curved to match exactly the curvature as the drum, such that each point on the print head is equidistant from the drum. That is, the radius of curvature of the drum, added to the perpendicular distance between the surface of the drum and the surface of the print head, equals the radius of curvature of the print head at every point on the print head. In this way, the electric field between the print head and the drum is theoretically identical at each point on the drum. However, the use of a print head curved to match the curvature of the drum causes problems as well. When such a print head is installed in an electrostatic printing machine, it is inevitably misaligned a small amount in a horizontal direction or in a skewed direction over the surface of the print head. Such misalignment may result at best in lower-quality printing, and at worst in physical interference between the print head and the drum that may damage either or both items.
The addition of more electrodes, modifications of the electronics, or differing hole sizes at discharge sites all mask the underlying problems of using a flat print head 100 with a curved drum 112, and can be cumbersome and expensive to implement. In addition, the use of a print head 100 having a curved surface with a radius of curvature identical to that of the drum 112 can result in interference between the two, and in practice does not obtain the results theoretically predicted for such a print head 112 due to inevitable errors in installation of the print head 100 into a printer where it is used. Thus, there is a need for a print head 100 capable of accurately depositing a substantially uniform charge onto a dielectric drum 112 that is tolerant of misalignment and other installation errors.
A curved print head is used for generating charged particles at a number of apertures and discharging those charged particles onto a cylindrical drum. The print head has a radius of curvature larger than the radius of curvature of the drum, thereby allowing the print head to accommodate errors in alignment resulting from installation or other factors. Further, stress on the dielectric within the print head is reduced by utilizing a shallower curvature on the print head. The difference in the radius of curvature of the print head and the drum is limited by the variation in electric field strength deposited on the drum across the width of the print head. That electric field variation may not exceed substantially fifteen percent from the center of the print head to either edge of the print head.